Process for producing crystalline tagatose

ABSTRACT

The present invention provides a method for producing tagatose crystals from an aqueous system using no organic solvent. In this method, seed crystals of tagatose are added to a tagatose solution in which a tagatose purity of solid contents in the solution is at least 70% and a solid contents concentration in the solution is 60 to 98 mass %, and crystallization of tagatose is carried out by stirring and cooling the solution while keeping a degree of super saturation of tagatose at 1.25 or lower. It is preferred that the solution containing tagatose is a solution containing no organic solvent. It is further preferred that a part of massecuite for which the crystallization is completed is added to a mother liquor so as to produce the tagatose crystals semi-continuously or continuously. It is furthermore preferred that the tagatose crystals formed from the massecuite are separated by centrifugation or filtration and dried to obtain tagatose crystals.

TECHNICAL FIELD

The present invention relates to a method for producing tagatosecrystals from a tagatose solution containing no organic solvent.

BACKGROUND ART

Tagatose is a carbohydrate obtainable by isomerization of galactose.However, its existing content in nature is very low, and itscharacteristics are seldom known other than a few properties such asdegree of sweetness, quality of taste and energy amount. The degree ofsweetness of tagatose is about 90% of sugar, and its quality of taste issimilar to fructose, and tagatose provides very fresh and sharpsweetness. Further, its energy amount is assumed to be 1.5 kcal/g, andit can be used as a low calorie sweetener.

As a method for producing tagatose crystals, a method wherein an organicsolvent such as ethanol is added to a high purity tagatose solution forcrystallization has been known. For example, Japanese InternationalPublication No. 5-504256 discloses a method for obtaining tagatosecrystals by adding seeds of tagatose crystals to a tagatose-containingsyrup which contains ethanol.

However, since detailed characteristics of tagatose e.g. solubility oftagatose in water have not been known, there has been no report about amethod for crystallization of tagatose from an aqueous system using noorganic solvent.

DISCLOSURE OF INVENTION

Accordingly, it is an object of the present invention to provide amethod for obtaining tagatose crystals from an aqueous system using noorganic solvent.

The present inventors have conducted extensive studies on physicalproperties of tagatose, and as a result, found that tagatose crystalscan be precipitated in a large amount by adding seed crystals to a highconcentration aqueous solution. The present inventors have accomplishedthe present invention based on this discovery.

Namely, the method for producing tagatose crystals of the presentinvention is characterized by adding seed crystals of tagatose to atagatose solution in which a tagatose purity of solid contents in thesolution is at least 70% and a solid contents concentration in thesolution is 60 to 98 mass %, and crystallizing tagatose by stirring andcooling the solution while keeping a degree of super saturation oftagatose at 1.25 or lower.

In the present invention, it is preferred that the solution containingtagatose is a solution containing no organic solvent. Further, it ispreferred that a part of massecuite for which the crystallization iscompleted is added to a mother liquor so as to produce the tagatosecrystals semi-continuously or continuously. Furthermore, it is preferredthat the tagatose crystals formed from the massecuite are separated bycentrifugation or filtration and dried to obtain tagatose crystals.

According to the present invention, it is possible to obtain tagatosecrystals without using organic solvent, by adding seed crystals oftagatose to a tagatose solution in which a tagatose purity of solidcontents in the solution is at least 70% and a solid contentsconcentration in the solution is 60 to 98 mass %, and crystallizingtagatose by stirring and cooling the solution while keeping a degree ofsuper saturation of tagatose at 1.25 or lower.

BEST MODE FOR CARRYING OUT THE INVENTION

As the solution containing tagatose to be used in the present invention,a tagatose solution in which a tagatose purity of solid contents in thesolution is at least 70% and a solid contents concentration in thesolution is 60 to 98 mass % (hereinafter simply referred to as tagatosesolution) , is used. In the present invention, preferred is a tagatosesolution in which a tagatose purity of solid contents in the solution isat least 90% and a solid contents concentration in the solution is 65 to95 mass %. If the tagatose content in solid contents is less than 70%,tagatose crystals can not be precipitated. Further, if the solidcontents concentration is less than 60 mass %, it is required to controlthe solution temperature at 20° C. or lower to form a saturatedsolution, whereby the energy costs are increased, such being notefficient. Moreover, if the solid contents concentration exceeds 98 mass%, the viscosity of the solution becomes too high, whereby the operationefficiency lowers and mimetic crystals are easily precipitated.

The tagatose solution can be obtained by known methods, and itspreparation method is not particularly restricted. For example, thetagatose solution can be prepared by a method wherein galactose isisomerized by using a soluble alkali metal salt or alkaline earth saltto produce tagatose (Japanese International Publication No. 5-504256), amethod wherein galactose is enzymatically isomerized by usingL-arabinose or isomerase to obtain tagatose (U.S. Pat. No. 6,057,135),etc.

Further, the tagatose solution is preferably the one deionized with ionexchange resins, but may contain salts or colorants within a range notaffecting the crystallization of tagatose.

Hereinafter, the method for producing tagatose crystals of the presentinvention will be described sequentially in order of steps.

1) Crystallization Operation

The above-mentioned tagatose solution is put in a container, and thesolution temperature is maintained at a range of 30 to 80° C. Then, seedcrystals of 0.006 to 80 mass % (tagatose crystals e.g. massecuitementioned below) is added, and while stirring the solution, the solutiontemperature is lowered at a rate of 0.1 to 10° C./hour until thesolution temperature reaches a range of 0 to 50° C., whereby tagatosecrystals are precipitated. In the present invention, it is necessary tomaintain the degree of supersaturation at 1.25 or lower during thecrystallization operation, and for this purpose, it is important toconduct the above-mentioned accurate temperature control. When thedegree of supersaturation is at 1.25 or lower, no particular problemwill be caused in the production of crystals. However, if the degree ofsupersaturation exceeds this range, mimetic crystals tend to form,whereby separation of crystals will be very difficult. In the presentinvention, it is more preferred to maintain the degree ofsupersaturation at 1.05 or lower in order to precipitate the crystalsuniformly in a large size. Here, the degree of supersaturation duringthe crystallization operation can be determined by sampling themassecuite, carrying out centrifugation at the temperature of themassecuite, and measuring the tagatose concentration of the supernatant.

2) Operation for Separating Crystals

Tagatose crystals can be separated by subjecting the solution(massecuite) for which crystallization operation is completed to e.g.centrifugation or filtration. Further, it is preferred to furtherwashing the separated tagatose crystals with a small amount of water,followed by drying. By such operation, tagatose crystals having a purityof at least 85% can be obtained.

In the present invention, the massecuite may be used as seed crystals.For example, to a fresh tagatose solution, massecuite is added in anamount of 5 to 80 mass % of the solution, and crystallization operationof the above step 1) is carried out, whereby production of tagatosecrystals can be conducted semi-continuously or continuously.

The structure of a crystallizer when the production of tagatose crystalsis conducted semi-continuously or continuously as mentioned above, isnot particularly restricted. As the crystallizer, crystallizers usefulfor the production of glucose crystals or fructose crystals may bementioned.

Hereinafter, the present invention will be described in detail withreference to examples. However, the present invention is by no meansrestricted to these examples.

TEST EXAMPLE (MEASUREMENT OF SOLUBILITY OF TAGATOSE)

To 10 ml of deionized water at 10° C., 2 g of tagatose crystals(manufactured by Sigma Aldrich Japan Co.) was added one by one, and atthe time when the tagatose crystals could no longer be dissolved, thetemperature was gradually raised, and the temperature when theundissolved crystals were completely dissolved was read. Then, further 2g of tagatose crystals was added, and same operation was repeated untilthe temperature reached around 70° C. The results are indicated in Table1.

TABLE 1 Tagatose concentration (mass %) Dissolving temperature (° C.)58.3 20.0 61.5 29.6 64.3 37.5 66.7 45.8 68.8 57.5 70.6 68.9

From Table 1, the solubility curve of tagatose to water is obtained, andfrom the ratio of the concentration of tagatose solution at temperature(A)/the solubility of tagatose at temperature (A), the degree ofsupersaturation is obtained.

EXAMPLE 1

In accordance with the method in Example 1 described in JapaneseInternational Publication No. 5-504256, steps from the isomerization tothe treatment with ion exchange resins were carried out.

Isomerization

2,500 g of galactose (manufactured by Wako Junyaku Kogyo K.K.) wasdissolved in 22,500 g of deionized water, and while stirring thesolution at 20° C. (liquid temperature) , 3,125 g of a slurry of calciumhydroxide (one obtained by dissolving 781 g of calcium hydroxide in2,344 g of deionized water) was gradually added, to conductisomerization. The progress of the reaction was checked by a HPLCanalysis every 30minutes. After 5 hours, at which the conversion ofgalactose reached 80% or higher, phosphoric acid was gradually added toadjust pH of the reaction solution to 5.8, to terminate the reaction.

Treatment with Ion Exchange Resins

By subjecting the reaction solution to centrifugation, the filtrate anddeposit were separately recovered. The deposit was washed with deionizedwater and then subjected to centrifugation again, and the recoveredsolid contents were mixed with the above filtrate. This mixed solutionwas allowed to pass through ion exchange resins (manufactured by OrganoCorporation) for deionization treatment, and then concentrated, toobtain 2082.5 g of a tagatose solution having a tagatose purity of 82%and a solid contents concentration of 86 mass %.

341.8 g of the tagatose solution was put in a separable flask having aninternal volume of 1,000 ml, and the solution temperature was adjustedto 68° C. while stirring in a water bath at 70° C. 0.03 g of seedcrystals of tagatose crystals was added to this solution, and thetemperature of the water bath was lowered at a rate of 2.5° C./hour, andat the time when the solution temperature reached 20° C., the solutionwas kept at this temperature for 5 hours.

The solution in the separable flask was used as massecuite containingcrystals of tagatose and the solid content concentration in the solutionportion in the massecuite was 78.7 mass %. The massecuite was subjectedto centrifugation, and the resulting crystals were washed with a smallamount of water, followed by drying, to obtain 211.3 g of tagatosecrystals having a purity of 92%.

EXAMPLE 2

340.5 g of the tagatose solution obtained in Example 1 was put in aseparable flask, and the same crystallization operation as in Example 1was carried out. After completion of the crystallization operation, 50 gof massecuite was left in the separable flask, and 210.2 g of thetagatose solution obtained in Example 1 was added, and stirring wascarried out for 15 minutes in a water bath of 70° C., and then thetemperature of the water bath was lowered at a rate of 2.5° C./hour, andat the time when the solution temperature reached 20° C., the solutionwas kept at this temperature for 5 hours.

The obtained massecuite was subjected to centrifugation, and theresulting crystals were washed with a small amount of water, to obtain205.8 g of tagatose crystals having a purity of 91%.

EXAMPLE 3

360.5 g of the tagatose solution obtained in Example 1 was put in aseparable flask having an internal volume of 1,000 ml, and the solutiontemperature was adjusted to 68° C. while stirring in a water bath of 70°C. 0.02 g of seed crystals of tagatose crystals was added to thissolution, and the temperature of the water bath was lowered at rates of0.5° C./hour for the initial 5 hours, and subsequently, 1° C./hour for 5hours, 1.5° C./hour for 5 hours, 2° C./hour for 5 hours and 2.5° C./hourfor 10 hours, and at the time when the solution temperature reached 20°C., the solution was kept at this temperature for 5 hours.

The solution in the separable flask was used as massecuite containingcrystals of tagatose, and this massecuite was subjected tocentrifugation, and the resulting crystals were washed with a smallamount of water, followed by drying, to obtain 195.6 g of tagatosecrystals having a purity of 95%.

Comparative Example

The filtrate separated by centrifugation in Example 3 had a tagatosepurity of 56%. This solution was concentrated to a solid contentconcentration of 84.8%, and the same crystallization as in Example 3 wasattempted. However, the added seed crystals were dissolved and noprecipitation of crystals was seen.

INDUSTRIAL APPLICABILITY

As explained above, according to the present invention, tagatosecrystals can be obtained without using an organic solvent, by addingseed crystals of tagatose to a tagatose solution in which a tagatosepurity of solid contents in the solution is at least 70% and a solidcontents concentration in the solution is 60 to 98 mass %, andcrystallizing tagatose by stirring and cooling the solution whilekeeping a degree of super saturation of tagatose at 1.25 or lower.

1. A method for producing tagatose crystals, characterized by addingseed crystals of tagatose to a tagatose solution in which a tagatosepurity of solid contents in the solution is at least 70% and a solidcontents concentration in the solution is 60 to 98 mass %, andcrystallizing tagatose by stirring and cooling the solution whilekeeping a degree of super saturation of tagatose at 1.25 or lower,wherein the solution containing tagatose is a solution containing noorganic solvent.
 2. The method for producing tagatose crystals accordingto claim 1, wherein a part of massecuite for which the crystallizationis completed is added to a mother liquor so as to produce the tagatosecrystals semi-continuously or continuously.
 3. The method for producingtagatose crystals according to claim 1, wherein tagatose crystals formedfrom the massecuite are separated from centrifugation or filtration anddried to obtain tagatose crystals.
 4. The method for producing tagatosecrystals according to claim 3, wherein a part of massecuite for whichthe crystallization is completed is added to a mother liquor so as toproduce the tagatose crystals semi-continuously or continuously.
 5. Themethod for producing tagatose crystals according to claim 4, whereintagatose crystals formed from the massecuite are separated fromcentrifugation or filtration and dried to obtain tagatose crystals.
 6. Amethod for producing tagatose crystals, characterized by adding seedcrystals of tagatose to a tagatose solution in which a tagatose purityof solid contents in the solution is at least 70% and a solid contentsconcentration in the solution is 60 to 98 mass %, and crystallizingtagatose by stirring and cooling the solution while keeping a degree ofsuper saturation of tagatose at 1.25 or lower, wherein a part ofmassecuite for which the crystallization is completed is added to amother liquor so as to produce the tagatose crystals semi-continuouslyor continuously.
 7. The method for producing tagatose crystals accordingto claim 6, wherein the solution containing tagatose is a solutioncontaining no organic solvent.
 8. The method for producing tagatosecrystals according to claim 6, wherein tagatose crystals formed from themassecuite are separated from centrifugation or filtration and dried toobtain tagatose crystals.
 9. A method for producing tagatose crystals,characterized by adding seed crystals of tagatose to a tagatose solutionin which a tagatose purity of solid contents in the solution is at least70% and a solid contents concentration in the solution is 60 to 98 mass%, and crystallizing tagatose by stirring and cooling the solution whilekeeping a degree of super saturation of tagatose at 1.25 or lower,wherein tagatose crystals formed from the massecuite are separated fromcentrifugation or filtration and dried to obtain tagatose crystals. 10.The method for producing tagatose crystals according to claim 9, whereinthe solution containing tagatose is a solution containing no organicsolvent.
 11. The method for producing tagatose crystals according toclaim 9, wherein a part of massecuite for which the crystallization iscompleted is added to a mother liquor so as to produce the tagatosecrystals semi-continuously or continuously.
 12. The method for producingtagatose crystals according to claim 10, wherein a part of massecuitefor which the crystallization is completed is added to a mother liquorso as to produce the tagatose crystals semi-continuously orcontinuously.
 13. The method for producing tagatose crystals accordingto claim 12, wherein tagatose crystals formed from the massecuite areseparated from centrifugation or filtration and dried to obtain tagatosecrystals.
 14. A method for producing tagatose crystals, characterized byadding seed crystals of tagatose to a tagatose solution in which atagatose purity of solid contents in the solution is at least 70% and asolid contents concentration in the solution is 60 to 98 mass %, andcrystallizing tagatose by stirring and cooling the solution whilekeeping a degree of super saturation of tagatose at 1.25 or lower,wherein the solution containing tagatose is a solution containing noorganic solvent, and wherein a part of massecuite for which thecrystallization is completed is added to a mother liquor so as toproduce the tagatose crystals semi-continuously or continuously.
 15. Themethod for producing tagatose crystals according to claim 14, whereintagatose crystals formed from the massecuite are separated fromcentrifugation or filtration and dried to obtain tagatose crystals. 16.A method for producing tagatose crystals, characterized by adding seedcrystals of tagatose to a tagatose solution in which a tagatose purityof solid contents in the solution is at least 70% and a solid contentsconcentration in the solution is 60 to 98 mass %, and crystallizingtagatose by stirring and cooling the solution while keeping a degree ofsuper saturation of tagatose at 1.25 or lower, wherein tagatose crystalsformed from a massecuite are separated from centrifugation or filtrationand dried to obtain tagatose crystals.
 17. The method for producingtagatose crystals according to claim 16, wherein the solution containingtagatose is a solution containing no organic solvent.
 18. The method forproducing tagatose crystals according to claim 16, wherein a part ofmassecuite for which the crystallization is completed is added to amother liquor so as to produce the tagatose crystals semi-continuouslyor continuously.
 19. The method for producing tagatose crystalsaccording to claim 16, wherein tagatose crystals formed from themassecuite are separated from centrifugation or filtration and dried toobtain tagatose crystals.